Point Defect

A point defect is a temporary or local deviation of the OTDR signal in the upward or downward direction. A point defect , or fault, can be reflective or non-reflective. It normally exhibits a loss of optical power. However, this condition may exhibit an apparent power gain. In some cases, it can even exhibit no loss or gain.

Refer back to the illustration on "OTDR trace of an installed cable plant" in the OTDR tutorial; it illustrates a reflective fault and a non reflective fault, both exhibiting loss. The picture below shows a non reflective fault with apparent gain and a reflective fault with no apparent loss or gain.

An OTDR trace showing a non reflective fault with apparent gain and a reflective fault with no apparent loss or gain.

Point-defects are located and measured using EIA/TIA-455-59. Test personnel must enter the appropriate input parameters including the source wavelength, the pulse duration, and the fiber or cable group index into the OTDR. The nature of fiber point-defects depends on the value of each parameter entered by the end user. The pulse duration usually limits the length of the point-defect while other input parameters, such as the wavelength, can vary its shape.

If the length of the fiber point-defect changes with the pulse duration, then the OTDR signal deviation is in fact a point defect. If the length remains the same, then the OTDR signal deviation is a region of high fiber attenuation. Regions of high fiber attenuation are referred to as attenuation nonuniformities.

Fiber point-defects occur from factory fiber splices or bends introduced during cable construction or installation. For shipboard applications, manufacturers are not allowed to splice fibers during cable construction. Fiber joints are natural sources of OTDR point defects. However, fiber breaks, cracks, ormicrobends introduced during cable installation are additional sources of point-defects.

Point-defects that occur at fiber joints are relatively easy to identify because the location of a fiber joint is generally known. A reflective or nonreflective fault occurs at a distance equal to fiber joint location. In most circumstances, an optical connector produces a reflective fault, while an optical spliceproduces a non reflective fault.

Reflective and non reflective faults occurring at distances other than fiber joint locations identify fiber breaks, cracks, or microbends. A fiber break produces a reflective fault because fiber breaks result in complete fiber separation. Fiber cracks and microbends generally produce non reflective faults.

A point-defect may exhibit apparent gain because the backscatter coefficient of the fiber present before the point-defect is higher than that of the fiber present after. Test personnel measure the signal loss or gain by positioning a pair of cursors, one on each side of the point defect.

The illustration below shows the positioning of the cursors for a point-defect showing an apparent signal gain. The trace after the point-defect is extrapolated as shown. The vertical distance between the two lines is the apparent gain of the point-defect.

Extrapolation for a point-defect showing an apparent signal gain.

Point-defects exhibiting gain in one direction will exhibit an exaggerated loss in the opposite direction. The picture below shows the apparent loss shown by the OTDR for the same point- defect shown in the picture above when measured in the opposite direction. Bidirectional measurements are conducted to cancel the effects of backscatter coefficient variations. Bidirectional averaging combines the two values to identify the true signal loss. Bidirectional averaging is possible only if test personnel have access to both ends of the test sample.

The exaggerated loss obtained at point-defects exhibiting gain in one direction by conducting the OTDR measurement in the opposite direction.

OTDRs can also measure the return loss of a point-defect. However, not all OTDRs are configured to make the measurement. To measure the return loss, the cursors are placed in the same places as for measuring the loss of the point-defect. The return loss is displayed when the return loss option is selected on the OTDR. The steps for selecting the return loss option depend upon the OTDR being used.

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